If to change any midi note, Ardour plays it only from the second time. From the first time it plays only the previous position of the notes.

Steps To Reproduce

1)make a midi track with any sounding instrument;
2)draw a midi note;
3)play it (can hear a sound);
4)change the note higher or lower;
5)play it (still can hear an unchanged sound);
6)play it again (only now can hear a changed sound)